Our previous two experiments have investigated the ability of weaned piglets to differentiate successfully between foods on the basis of the protein (CP) content and also on the basis of the lysine content, when offered a choice of two feeds differing in levels of either protein or lysine but with levels of other nutrients kept constant (Dalby et al. 1994a; Dalby et al. 1994b). This experiment was designed to test whether weaned piglets are able to successfully select a diet if both the levels of CP and lysine differ in the foods on offer. This was investigated by offering a choice of three foods containing low and high levels of both CP and lysine, such that it was unlikely that a combination of any two foods would satisfy the piglet in relation to both its CP and lysine requirement.